Echo cancelers are utilized in a variety of applications for canceling the effects of a reflected signal that results from the undesired coupling of a device's output signal (electrical or acoustic) back to its input signal path. Functionally, an echo canceler receives both the original reference signal and the reflected signal and attempts to approximate what the reflected signal will be from the original reference signal. The echo canceler subtracts this approximated reflected signal from the actual reflected signal to obtain an error signal. Ideally, this error signal is zero when the echo canceler has successfully approximated the reflected signal. Accordingly, no reflected signal will be returned back to the transmitting source as desired.
In typical telephone communication systems, some reflected signals result at the interface between two-wire lines systems such as the PSTN and four-wire line systems such as mobile or cellular communication systems. Some typical echo cancellation techniques provide predetermined fixed echo cancellation signals in the receive path of a four-wire line system. However, each connection to the PSTN is different resulting in a different impedance which results in different refection characteristics. As a result, fixed and predetermined echo cancellation techniques do not adequately suppress reflected signals.
Some other typical echo cancellation techniques continually change the echo cancellation signal during a telephone call in an attempt to compensate for the characteristics of that specific connection to the PSTN. However, because the reflected (echo) signal includes the voice signals, it is impossible to accurately estimate the reflection characteristics of the interface.
Accordingly, what is needed are an improved apparatus and method for suppressing signals that are reflected at the interface between two-wire line and four-wire line systems. What is also needed are an apparatus and method for canceling reflected signals that takes into account the characteristics of the connection to the PSTN. What is also needed are an apparatus and method that cancels reflected signals without interference from the voice communication.